In many cell types, store-operated Ca2+ entry represents the primary, if not sole, mechanism underlying long-lasting elevations in intracellular Ca2+ that follow IP3-mediated release of Ca2+ from intracellular stores. Previous investigations have identified STIM1 as the sensor for ER luminal Ca2+ concentration and CRACM1 (or Orail) as the Calcium Release-Activated Calcium (CRAC) channels in the plasma membrane. Upon Ca2+ depletion of stores, STIM1 translocates into vesicular structures (puncta) underneath the plasma membrane, where it may bind to and activate multimeric assemblies of CRACM1. When overexpressed jointly, but not individually, STIM1 and CRACM1 reconstitute large CRAC currents. There are three mammalian homologous CRAC channel proteins: CRACM1, CRACM2, and CRACM3. CRACM2 has been shown to enhance store-operated Ca2+ entry and produce large CRAC currents when co-expressed with STIM1. Although the same study found that CRACM3 does not enhance store-operated Ca2+ entry and no currents have been observed, the protein apparently restored store-operated Ca2+ entry to normal levels when CRACM1 was knocked down by siRNA.
Store-operated calcium entry is a ubiquitous element of cell function, and CRAC channels are vital to a wide variety of cellular processes. CRAC channel homologs exhibit distinct electrophysiological and pharmacological properties, and thus a need exists for methods and assays for determining the identity of the CRAC channels contributing to Icrac activity in different cells.